(a) Field of the Invention
The present invention relates generally to a communication signal modulation and demodulation technique and, more particularly, to a swept frequency modulation and demodulation technique using a channel equalizer.
(b) Description of Related Art
Communication systems and, particularly, mobile cellular communication systems such as telephone systems, typically use frequency division multiple access (FDMA) and/or time division multiple access (TDMA) techniques to establish a plurality of communication channels between a base station and a multiplicity of users, such as mobile units, located within a particular geographic region. As is well known, an FDMA technique multiplexes a signal into one of a number of different frequencies or frequency regions while a TDMA technique multiplexes a signal into one of a number of repeating time slots associated with a frequency band. To maximize the number of channels for an available frequency band, some mobile digital communication systems, such as digital telephony systems, combine FDMA and TDMA techniques to establish a multiplicity of channels, wherein each channel uses one of a number of repeating time slots at one of a number of different frequencies.
Beyond a few hundred meters, mobile cellular communication systems that operate in, for example, the 800 MHz to 900 MHz and the 1800 MHz to 2000 MHz frequency regions begin to undergo degradation caused by multipath. Multipath largely results from the presence of multiple reflectors scattered throughout a geographical region in which a communication signal is transmitted. These multiple reflectors cause the transmitted signal to take numerous, different-length paths between the transmitter and a receiver unit which, in turn, causes numerous phase and time delayed versions of the transmitted signal to reach the input of the receiver unit simultaneously. The over-all range of time delays is commonly called the delay-spread of the channel. In the presence of multipath, with no direct line of sight path, the receiver unit collects a signal having an amplitude with a Rayleigh distribution. As a result, in the absence of a direct path, the receiver unit experiences deep fading or signal nulls with a high degree of certainty. Generally such nulls may last for fractions of a second, seconds, or minutes depending upon the speed of the mobile terminal relative to the base station which, in turn, determines how fast the channel is changing.
In the past, fading has been reduced using energy dispersal techniques in the form of frequency diversity. Energy dispersal techniques, such as frequency hopping (FH), code division multiple access (CDMA), and other spread spectrum techniques spread the energy associated with a signal over a number of different frequencies to form a channel having an overall response that is a function of the response at each of the different frequencies (in both the amplitude and phase domains). In effect, an energy dispersal technique averages the amplitude and phase response at all of the frequencies being used to prevent a receiver unit from being limited to the response at only one of the frequencies and, thereby, to prevent the receiver unit from experiencing reduced effectiveness because it is located within an effective null at a particular frequency. However, some energy dispersal techniques such as slow or fast FH lose coherency from hop to hop which requires additional processing and increases the overhead associated with the transmission of a signal.
Regardless of the specific modulation technique chosen, it is always an objective in cellular wireless communications to accommodate many users at the same time. Therefore, whatever pattern of modulation one transmitter-receiver pair uses, it is necessary to accommodate a multitude of users, each one assigned to a specific modulation pattern. When the modulation patterns (e.g., the time-frequency tracks) for any two users do not ever overlap, the access technique is called orthogonal. FDMA and TDMA are examples of orthogonal modulation. It is possible, in some situations, to make spread spectrum signals orthogonal even though the time-frequency tracks overlap at some point. More commonly however, overlaps cannot be avoided in which case, the receiver contains what is known as self-noise.
The near/far problem is closely associated with self-noise in that it is also a characteristic of non-orthogonal spread spectrum techniques. As is generally known, a near/far problem arises when a receiver unit tries to receive and tune to a signal being transmitted by a transmitter located at a relatively "far" distance, but is incapable of doing so because the receiver tuning circuitry is overwhelmed or captured by a signal being transmitted by a transmitter located at a relatively "near" distance. In effect, the power of the "near" signal causes the receiver unit to be incapable of distinguishing or decoding the lower power "far" signal. The near/far problem necessitates the use of real time power control.
While, the previous discussion emphasizes the effect of multipath in the creation of a fade condition at one or more frequencies, multipath also creates a condition, due to delay-spread, where modulation symbols are smeared into one another. This condition is known as inter-symbol interference (ISI) and is a time-based phenomenon rather than a frequency-based phenomenon. By making modulation symbols much longer than any anticipated delay-spread, one can minimize ISI by confining it to a small region of time near the modulation symbol transitions. This procedure does not eliminate fading however. Thus, while it is a fairly simple matter to use energy dispersal to compensate for fading when the intersymbol time of a digital communication channel is much longer than the delay-spread, because of ISI, it is more difficult to compensate for fading using energy dispersal when the intersymbol time is reduced to be on the order of or to be lower than the delay-spread. Unfortunately, due to higher usage and throughput requirements, newer communication systems are being designed to have higher data rates and, therefore, shorter intersymbol times which, in turn, makes these systems more susceptible to ISI.
In the past, equalization techniques have been used to compensate for intersymbol interference. Generally, equalization techniques try to calculate or configure a series of tap weights associated with a tapped delay line or a digital filter, such as a Kalman filter, in a manner that best eliminates the effects of a channel on a received signal. Typical equalization techniques are discussed in Qureshi, "Adaptive Equalization," Proceedings of the IEEE (1985); Reprinted in, Rappaport ed., "Selected Readings in Cellular Radio and Personal Communications," IEEE ISBN 0-7803-2283-5.COPYRGT. (1994), and may use an iterative or adaptive routine designed to calculate tap weights that minimize, for example, the mean squared error, between a known test sequence and the test sequence after that test sequence has been transmitted through a channel and equalized. However, these equalization or tap weight estimation routines usually require a lot of processing over a number of iterations, use filters with only a small number of tap weights (e.g. three), and always require that a known training or test signal be sent through the channel on a periodic basis.
Other methods of reducing fading and/or intersymbol interference include the use of forward error correction (FEC) coding, directive antennas, and M-ary modulation techniques. While FEC coding can correct a certain number of transmission bit errors, FEC coding increases the transmitted bit rate which may further increase the intersymbol interference and/or cause longer decoding delays. Likewise, although directive antennas decrease fading by assuring that less multipath is received, directive antennas are not always possible or easy to employ in cellular communication systems, such as mobile telephone systems, because directional antennas are physically much larger structures than omni-directional antennas, and to be useful, one must know the direction to point them. Also, while M-ary modulation techniques are able to increase the symbol length and, therefore, decrease intersymbol interference without decreasing throughput, M-ary techniques require an increase in transmitted power and are more susceptible to noise within the channel.
It is desirable, therefore, to provide a coherent frequency modulation technique that allows a high symbol transmission rate while avoiding fading and intersymbol interference. One relatively coherent energy dispersal method discussed in Humblet et al., "A Multiaccess Protocol for High-Speed WLAN," VTC '96, Atlanta, Ga. (April 1996), and Chelouche et al., "Digital Wireless Broadband Corporate and Private Networks: RNET Concepts and Applications," IEEE Communications Magazine, pp. 42-51 (January 1997) applies a frequency-ramped sweep signal to a modulated communication signal to produce one of a number of track signals which is then transmitted through a wireless channel. A receiver unit associated with this system receives the track signal, removes the swept frequency modulation from the received track signal and then demodulates the communication signal using standard techniques.
While the swept frequency modulation technique discussed in these articles provides generally coherent energy dispersal, this system still has a number of problems. For example, although this system sends multiple swept frequency signals or tracks over a wireless channel in a simultaneous manner, each of the tracks includes a single, fixed-frequency header having synchronization or other information pertaining to the track therein. Because the non-swept header must be sent through the channel for a discrete amount of time, the number of tracks that can be simultaneously multiplexed within the system is limited by both the time width of the fixed-frequency headers and the frequency width of the swept frequency portions of the tracks. In most cases, due to the length of the headers, the swept frequency tracks of this system cannot be placed directly adjacent one another in the swept frequency/time domain and, therefore, this system does not use all of the available frequency/time space associated with the particular frequency range over which a communication signal is being swept. Also, in this system, there is a discrete frequency jump between the end of the header and the start of the ramped frequency portion of the track which complicates the demodulation of these signals because a demodulator must track phase jumps and/or must demodulate the fixed-frequency header and the ramped frequency portion of each track independently.
Furthermore, using the swept frequency modulation scheme disclosed in these articles introduces a rapid change in the amplitude and phase of the received signal which mimics the transfer function of the multipath channel, H(jw). It is just this amplitude variation that provides the desired anti-fading property of this type of signal. However, if not compensated for, the amplitude and phase variations make the signal difficult to demodulate using a conventional receiver. In particular, the swept frequency signal picks up amplitude gains and phase shifts across an entire band of frequencies, not just one. If not removed, these time-varying impairments degrade or prevent detection and demodulation of the underlying modulated communication signal because a conventional demodulator will have a problem automatic gain controlling (AGC) and phase locking the received signal within the very short period of time that the demodulator receives the transmitted signal, i.e., the sweep frame or sweep interval associated with each track. The above-identified articles fail to recognize or address this problem but, instead, merely assume that the frequency sweep component of a swept frequency signal can be totally compensated using a downconversion signal also containing the linear sweep.